The present invention relates to a digital control system and a digital temperature compensated crystal oscillator (DTCXO).
Recent advances in IC technology promote size reduction and cost reduction in equipment for mobile communication such as devices or telephone sets for portable, mobile and cordless 0 telephone systems. This progress increases the number of subscribers and users of the radio communication systems, and the carrier frequency spacing (12.5 KHz, for example) and the modulation band width (5 KHz, for example) are decreased. Therefore, a source of oscillation frequency is required to fulfill a severe requirement of the frequency stability. For example, the frequency stability must be equal to or lower than 1 ppm; .vertline..DELTA.f/f.vertline..ltoreq.1 ppm.
In a conventional TCXO (temperature compensated crystal oscillator), a temperature compensating network having a thermistor, a resistor and a capacitor is connected in series to a crystal unit, and designed to hold the output frequency approximately constant notwithstanding changes in the temperature of the environment by controlling the impedance of the circuit. This oscillator can be constituted by uncostly analog component parts, but its production process is not easy when a unit to unit variation of each component is taken into account. Specifically when the demand for the frequency stability is changing from a conventional level of .+-.2.5 ppm to a recent level of .+-.1.0 ppm with the recent trend toward a narrower radio frequency channel spacing, it is becoming more difficult to meet the requirement of a high precision level with the conventional analog type TCXO design.
On the other hand, there is known a digital temperature compensated crystal oscillator (DTCXO) composed of a temperature sensor, an A/D converter, a memory section, a D/A converter and a voltage controlled crystal oscillator (VCXO). Data for temperature compensation fitted to the frequency-temperature characteristic of the VCXO is preliminarily stored (or programmed) in the memory such as a ROM. A control voltage is determined from the stored data in accordance with the sensed temperature of the surroundings, and applied to the VCXO to hold the output frequency constant. Without the need for complicated combination of component parts as in the analog type TCXO, the DTCXO is suitable for large scale integration (LSI) and automatic production, and promising for size reduction and cost reduction.